1. Field of the Invention
The present invention relates to a wireless access system, and more particularly, to a method for configuring system information and a method for transmitting system information, when a subframe structure is used. The present invention also relates to a method for notifying resource ratio information, when a subframe structure is used.
2. Discussion of the Related Art
A typical frame structure used in a wireless access system will be described below.
FIG. 1 illustrates a frame structure in a broadband wireless access system (e.g. Institute of Electrical and Electronics Engineers (IEEE) 802.16).
Referring to FIG. 1, the horizontal axis represents Orthogonal Frequency Division Multiple Access (OFDMA) symbols as time units and the vertical axis represents the logical numbers of subcarriers as frequency units, in a frame. One frame is divided into data sequence channels each having a predetermined duration according to the physical characteristics of the data sequence channels. Specifically, a frame is divided into a DownLink (DL) subframe and an UpLink (UL) subframe. A Transmit/receive Transition Gap (TTG) is interposed between a DL subframe and a UL subframe and a Receive/transmit Transition Gap (RTG) is interposed between frames.
A DL subframe may carry a preamble, a Frame Control Header (FCH), a DL-MAP, a UL-MAP, and one or more DL data bursts. A UL subframe may carry one or more UL data bursts and a ranging subchannel.
The preamble is a predetermined data sequence residing in the first symbol of each frame, for use in a Mobile Station's (MS's) acquisition of synchronization with a Base Station (BS) and channel estimation. The FCH provides information about channel allocation and channel coding of the DL-MAP. The DL-MAP and the UL-MAP are Media Access Control (MAC) messages carrying channel resource assignments to MSs. The DL data bursts and the UL data bursts are data units that the BS transmits to MSs or MSs transmit to the BS, respectively.
A Downlink Channel Descriptor (DCD) that can be transmitted in the frame structure illustrated in FIG. is a MAC message describing the physical characteristics of a downlink channel and an Uplink Channel Descriptor (UCD) is a MAC message describing the physical characteristics of an uplink channel.
On a downlink, an MS may detect a preamble transmitted from a BS and then decode a DL-MAP using information acquired from an FCH. The BS may transmit scheduling information to MSs in every frame (e.g. every 5 ms) by a DL-MAP or UL-MAP message to allocate downlink or uplink resources to the MSs.
[Table 1] below illustrates an exemplary format of the DL-MAP message.
TABLE 1SizeSyntaxin bitsDescriptionDL-MAP_Message_Format( ){—— Management Message Type = 28— PHY Synchannel FieldVariablesee appropriate PHYspecification. DCD Count8— Base station ID48 — Begin PHY-specific—see applicable PHY subclausesection{  if(Wireless MAN-OFDMA){   No.OFDMA symbols8Number of OFDMA symbolsin the DL subframe includingall AAS/permutation zoneand including the preamble  }——  for(i=1; i<=n; i++)}—For each UL-MAP element1 to n.   DL-MAP_IE( )variableSee corresponding PHYspecification  }—— }—— if!(byte boundary)}——  Padding Nibble4Padding to reach byteboundary }——}——
Referring to [Table 1], Management Message Type is 2, identifying a DL-MAP message. A DCD provides downlink channel information (e.g. downlink burst profiles). DCD Count field specifies a value of the Configuration Change Count of the DCD. Hence, DCD Count field may indicate whether the DCD has been changed. No. OFDMA symbols filed specifies the number of OFDMA symbols allocated to a DL subframe. The DL-MAP message may include a variety of DL-MAP Information Elements (IEs) (i.e. DL-MAP_IE( ).
[Table 2] below illustrates an exemplary format of the UL-MAP message.
TABLE 2SizeSyntaxin bitsDescriptionUL-MAP_Message_Format( ){—— Management Message Type = 38— Reserved8Shall be set to zero UCD Count8— Allocation Start Time32 — Begin PHY-specific—see applicable PHYsection{subclause  if(Wireless MAN-——OFDMA){   No.OFDMA symbols8Number of OFDMA sym-bols in the UL subframe  }——  for(i=1 ; i<=n; i++){—For each UL-MAPelement 1 to n.   UL-MAP__IE( )variableSee corresponding PHYspecification  }—— }—— if !(byte boundray){——  Padding Nibble4Padding to reach byteboundary }——}——
Referring to [Table 2], Management Message Type is 3, identifying a UL-MAP message. A UCD provides uplink channel information (e.g. uplink burst profiles). UCD Count field specifies a value of the Configuration Change Count of the UCD. Hence, UCD Count field may indicate whether the UCD has been changed. Allocation Start Time specifies an effective start time of uplink allocation defined by the UL-MAP message, in PHY-specific units. No. OFDMA symbols field specifies the number of OFDMA symbols allocated to a UL subframe.
In a Wireless Metropolitan Area Network-Orthogonal Frequency Division Multiple Access (WMAN-OFDMA) system, a BS may transmit most of system IEs (or system configuration information) to MSs periodically (e.g. every 0.5 to 2 seconds or every up to 10 seconds) via a DCD/UCD message. In addition, the BS may transmit certain system configuration information, such as a frame number, a frame duration, a BS Identifier (ID), the number of symbols, to the MSs via a MAP message.
In general, system IEs are transmitted with a long period and an MS needs to acquire the system IEs in order to enter a network. Thus, the network entry of the MS may take a long time. In addition, since system information carried in a DL-MAP does not change often, it is not necessary to transmit the system information in every frame.
System information delivered by a DCD/UCD takes the form of Type, Length, and Value (TLV). Accordingly, additional information such as Type and Length is required for a specific system IE, thus consuming resources. The length of a message is actually determined based on TLV encoding information of system IEs included in the message. Therefore, the BS should notify an MS of the size of resources occupied by the message all the time.
A system IE may be transmitted at a fixed size. Because the system IE includes information of a fixed size, the BS does not need to transmit resource allocation information about the Type and Length fields of the system IE to MSs. If all system IEs are used at fixed sizes, the scalability of the system IEs may be decreased. Moreover, all system information delivery messages are transmitted at fixed sizes every fixed period and thus more radio resources are used to transmit system information.
If the BS transmits a system information delivery message including system IEs of fixed sizes every predetermined period, which is longer than a superframe period (e.g. a multiple of the superframe period, such as 40 ms, 80 ms, etc.), less resources may be used than when system information is transmitted every superframe period. However, the problems of decreased scalability and delayed initial network entry of MSs still exist.
One system IE used in a WMAN-OFDMA Time Division Duplexing (TDD) system (e.g. IEEE 802.16d/e/Rev2) is a DL/UL ratio field indicating the ratio between DL subframes and UL subframes (or No. OFDMA symbols).
The DL/UL ratio field may be included in the DL-MAP and UL-MAP messages. Once a DL/UL ratio is set, the system does not change the DL/UL ratio often. Especially if information about the configuration and/or length (e.g. the number of Orthogonal Frequency Division Multiplexing (OFDM) symbols) of a subframe can be acquired from system information in a system using superframe and subframe structures, it is a waste of radio resources to transmit DL/UL ratio information on a symbol basis.